In recent years, a reduction in fuel consumption of automobiles has been increasingly demanded in connection with a demand for energy conservation. In order to deal with such a demand, a further reduction in rolling resistance has been desired for tires. A method of reducing the rolling resistance of tires by optimizing the tire structure has been studied. A method using a rubber composition exhibiting low heat build-up has been most generally employed.
In order to obtain a rubber composition exhibiting low heat build-up, a modified rubber has been extensively developed for a rubber composition using silica or carbon black as a filler. In particular, a method of modifying the polymerization active terminal of a conjugated diene polymer obtained by anionic polymerization using an organolithium compound with an alkoxysilane derivative having a functional group which interacts with the filler has been proposed as an effective method.
However, this method is generally applied to a polymer of which the polymer end exhibits living properties. Specifically, an improvement in modification of cis-1,4-polybutadiene particularly important for tire side wall rubber, tire tread rubber, and the like is limited. Moreover, a modification effect on a rubber composition containing silica or carbon black is not necessarily sufficient. In particular, a modification effect on cis-1,4-polybutadiene containing carbon black has been achieved to only a small extent.
In order to overcome the above-mentioned drawback, a method which reacts the active terminal of a conjugated diene polymer having a high cis content obtained using a rare earth catalyst with a functional group-containing alkoxysilane derivative which interacts with a filler to obtain an end-modified conjugated diene polymer, and a method which adds a condensation accelerator to a reaction system at the time of alkoxysilane modification have been proposed (see Patent Documents 1 and 2). However, a further improvement in performance of the resulting modified conjugated diene polymer has been desired.
Patent Document 1: WO 03/046020
Patent Document 2: JP-A-2005-8870